It is well-established that NAbs emerge in most patients within a few months following HIV infection and that the NAbs developed by individual patients primarily target the autologous virus. However HIV+ sera can also neutralize heterologous isolates (cross-neutralizing antibody responses). The epitope-specificity of these cross-neutralizing antibody responses is largely unknown. We examined HIV+ antiretroviral-naive patients and identified a small number of subjects that developed broad anti-HIV plasma cross-neutralizing antibody responses. The plasma of one patient neutralizes all isolated tested against, irrespective of clade. Epitope- mapping analysis indicated that the exceptionally broad plasma neutralizing activity of that subject is exclusively due to anti-CD4-BS antibodies. However, because most patients, irrespective of the breadth of their plasma cross-neutralizing antibody responses, develop anti- CD4-BS antibodies, we hypothesize that the epitope(s) recognized by that patient's anti-CD4- BS antibodies differ from those recognized by the anti-CD4-BS antibodies of most HIV+- patients. We propose to isolate and characterize anti-CD4-BS MAbs from that patient and also to examine how the virus manages to escape their action. We have also identified another patient that developed 4E10-like and 2F5-like antibodies. We propose to map the precise epitope of these antibodies and examine the viral determinants of escape from those antibodies in that patient. In most cases, however, we were unable to map the epitope-specificities of broad plasma cross-neutralizing antibody responses to a particular epitope on Env. Our proposed studies aim at identifying the epitopes recognized by broadly cross-reactive NAbs developed during natural HIV-infection. This analysis will provide insights into the pathways that lead to the development of cross-reactive NAbs during HIV-infection, their epitopes, and the determinants of viral-escape from broadly-cross-reactive NAbs. Information from such studies can be used to develop more effective immunogens.